This invention relates to the art of injection blow molding, and more particularly relates to injection blow molding methods and apparatus suitable for producing molecular biaxially oriented hollow articles from thermoplastic materials.
It is well known to those skilled in the art that molecular orientation substantially improves the stiffness, ultimate tensile strength, yield strength, impact resistance, clarity, and permeation resistance of many thermoplastic materials. Molecularly oriented articles having these improvements have great commercial significance. For example, high strength plastic strapping is that which possesses a high degree of molecular orientation in its length direction by virtue of having been uniaxially stretched, at the polymer orientation temperature, in the machine direction. "Mylar" plastic film and sheet produced by the DuPont Company possesses outstanding strength properties and permeation resistance as a result of inherent high levels of biaxial molecular orientation, molecular orientation along two perpendicular axes of the film or sheet. In this case the film or sheet is oriented in the so-called machine direction, in which it is originally extruded and stretched, and then oriented by stretching in the cross machine direction. The degree of molecular orientation and the strength of the film or sheet in either of its two perpendicular directions is directly proportional to the amount of stretching imposed upon it in that particular direction. Molecular orientation may be either balanced or unbalanced in the two directions depending upon whether the film or sheet has been stretched equally or unequally in each direction.
Another commercially important plastic product which is biaxially oriented is the so-called "PET" plastic carbonated soft drink bottle which is produced from polyethylene terephthalate. The composition, physical and other properties of this bottle are typical of that taught in U.S. Pat. No. 3,733,309 issued May 15, 1973 and entitled "Biaxially Oriented Poly (Ethylene Terephthalate) Bottle." The majority of the biaxially oriented PET beverage bottles are presently being produced by the so-called two stage "reheat blow" method, using a separate machine for each stage. In a first stage injection molding machine, PET parisons or preforms are first injection molded in a cooled mold, at melt temperatures of about 540.degree. F., which is above the polymers melting point, and then cooled down and removed from the injection molding machine for later use as feed stock to a separate second stage reheat-blow machine, where the biaxially oriented bottle is produced. Upon entering the reheat blow machine, the cold parisons, whose shape resembles that of a test tube having a threaded bottle neck finish at its open ended top, are heated uniformily in an oven to its orientation temperature, which for PET is about 190.degree. F. to 200.degree. F. (which is below PET's melting point). The temperature conditioned parisons are then placed within cooled bottle blow molds which clamp the parisons by their necks upon closing of the blow molds. Metal pushrods are then inserted and pushed into the parisons through their open necks, and the parisons, whose initial lengths are shorter than that of the finished bottle, are stretched axially against the bottom of the blow molds to their final lengths, thereby effecting axial or longitudinal orientation. Radial or so-called "hoop direction" orientation is next achieved by introducing compressed air inside the axially stretched parisons to expand them outward and into contact with the cooled surfaces of the bottle blow molds. After cooling sufficiently for subsequent handling, the blow molds open and the biaxially oriented bottles are ejected from the machine. While this method is suitable for use with orientable thermoplastic, it requires a substantial capital investment in the injection molding machine and the reheat blow machine and its associated parison transfer equipment and heating ovens. Furthermore, a considerable amount of energy is consumed in reheating the cold parisons in the oven, which adds to the cost of the finished oriented articles.
It has long been recognized that the reheating step can be avoided if a so-called in-line single stage injection blow molding process were utilized to make biaxially oriented hollow articles using a single "hot parison" injection stretch-blow molding machine. In the "hot parison" in-line method, the parison is formed by injection molding, cooled to orientation temperature, and then stretched axially and blown radially to its final product shape, without ever being allowed to cool to room temperature. A number of such in-line "hot parison" injection stretch blowing methods and apparatus have been disclosed in the patent literature and as such constitute the prior art.
As set forth in U.S. Pat. No. 3,470,282 issued Sept. 30, 1969 to A. J. Scalora, a hot thermoplastic parison is first formed by injection molding the thermoplastic material, at a temperature above its melting point, over a generally cylindrical core, called an inner sleeve, which is positioned in the female cavity of an injection mold. The parison is then cooled, while on the core and within the injection mold, by suitable cooling means located therein, down to a narrow temperature range, which includes the preferred orientation temperature of the material being processed, said temperature range being relatively uniform and covering all points across the thickness and at the surfaces of the parison, and said temperature range also being below the thermoplastic materials homogenous melting temperature. The narrow temperature range for PET parisons would be about 190.degree. F. to 200.degree. F. After reaching its narrow orientation temperature range, the uniformly cooled parison is then removed from the injection mold and transferred, while still on the inner core, to a blow mold having cooling means therein. While in transit, or after being positioned within the closed blow molds, the parison is stretched axially by the outward extension of a valve located within the inner core over which the parison had been previously molded. Next, the parison is inflated, while positioned within the blow molds, thus stretching the parison along a second axis which is perpendicular to the longitudinal axis or direction of axial stretching. Stretching the thermoplastic parison at its orientation temperature, by longitudinal or axial extension of a valve within the core rod, and by radial inflation, sometimes referred to as "hoop stretching," yields a biaxially oriented hollow article.
The arrangement described above has the virtues of simplicity and energy conservation mentioned previously, but it can not operate at the high production rates necessary for economical production. For example, the parison must first be brought to orientation temperature throughout its entire thickness. If the metal surfaces of the core and injection mold cavity are maintained at temperatures at or slightly below the orientation temperature range of the thermoplastic to be processed, 190.degree. F. to 200.degree. F. for PET, the parison will eventually be cooled to an equilibrium temperature condition corresponding to the desired orientation temperature range, across its thickness, while it is still in the injection mold. However, the rate of cooling of the parison within the injection mold will be extremely slow because of the small temperature differential between the parison, the core, and the mold surfaces. Thus, the speed of operation of the apparatus will be limited by the long injection molding cycle required. In contrast, if the core and injection mold cavities are maintained at a much lower temperature, conventionally about 35.degree. to 40.degree. F. for PET, the rate of cooling will be increased substantially, but an uneven temperature distribution will be created across the thickness of the parison. Such rapid cooling of the parison, if accomplished within an economical and commercially feasible cycle time, will result in surface temperatures of the parison which are substantially below the orientation temperature range of the thermoplastic being processed and will actually approach the temperature of the core and the injection mold, while the middle or mid-point of the parison walls will be substantially above the desired orientation temperature range. Therefore, satisfactory orientation will not be achieved during the stretching and blowing of parisons which have such substantial mal-distributions of temperatures across their thickness, major portions of which lie outside the orientation temperature range of the thermoplastic being processed.
The cycle time limitations resulting from the slow parison or preform cooling inherent in Scalora's teaching are overcome to a certain degree in other subsequently disclosed art. For example, in U.S. Pat. Nos. 3,966,378 and 4,151,248, issued on June 29, 1976 and on Apr. 24, 1979 respectively, to Emery I. Valyi.
In U.S. Pat. No. 3,966,378, a parison is formed on a first core in an injection mold, cooled in the injection mold, transferred on said first core to a pre-blow mold and partially expanded against the inner surfaces of the pre-blow mold to a shape intermediate that of the parison and that of the finished article. Next, cooling is effected on the first core in the pre-blow mold to a uniform temperature across its thickness within the desired orientation temperature range of the thermoplastic being molded. The temperature conditioned parison is then transferred to a second blow core and later transferred to a third mold where it is axially stretched by said second core and finally radially stretched and expanded in the third mold, which is the final blow mold, to form a hollow biaxially oriented article. Separate parisons may be simultaneously injection molded, preblown, and cooled to orientation temperature, and finally stretch-blown, if multiple sets of cores and molds are utilized. Because the parison is not conditioned to a uniform orientation temperature across its thickness in the injection mold, the injection mold can operate at a reasonable and economical production rate. However, the necessity for additional preblow molds and additional cores and transfer means greatly complicates the apparatus and requires greater capital investment. Furthermore, pre-blowing to an intermediate shape is actually somewhat self defeating in that it sacrifices the amount of orientation which may be subsequently imparted to the parison, since the degree of orientation which may be imposed is directly proportional to the amount of stretch which takes place after the parison has been brought to the desired orientation temperature, which in this case takes place in the pre-blow mold. Obviously, the amount of orientation-stretching which can be accomplished from stretching the parison's intermediate shape to its final shape is less than if the parison was stretched, at orientation temperature, from its original shape to its final shape.
In U.S. Pat. No. 4,151,248, Valyi avoids the need for pre-blow molds with its attendant sacrifice in the levels of orientation which may be achieved, by claiming a method for preparing hollow oriented plastic articles wherein a parison is formed and cooled rapidly on a first core in an injection mold to an average temperature suited for orientation but having unequal distribution of temperature across the walls of said parison, being cold on the outer surfaces and hot in the middle. Next, the cooled parison on said first core is transferred to a tempering mold where it is stripped from said first core and deposited in the tempering mold. The cooled parison is then conditioned or tempered in the temperature controlled tempering mold to equalize the temperature distribution across the walls of the parison and attain a uniform temperature distribution corresponding to the desired orientation temperature of the thermoplastic material being molded. The tempering is aided by insertion of a separate stretch-blow core into and against the parison, to provide pressure contact between the parison and the tempering mold, thereby speeding heat transfer between the two. The temperature conditioned parison is then transferred on the stretch-blow core to a third mold, which is the stretch-blow mold, and is finally axially stretched by telescoping extension of said stretch-blow core, and then radially expanded and cooled in said stretch-blow mold, to form a biaxially oriented hollow article. Because the parison need not be conditioned to a uniform orientation temperature across its wall thickness in the injection mold, the parison may be removed from the injection mold early, and the injection molding step can be operated at a reasonable rate, and much faster than otherwise would be possible. However, the necessity for additional cores, molds, and transfer means greatly complicates the apparatus and substantially adds to the costs.
In U.S. Pat. No. 3,776,991, issued Dec. 4, 1973, Marcus teaches a method for producing biaxially oriented hollow plastic articles in a rotary type injection molding machine having a least four stations, wherein a parison is formed on a first core within an injection mold at the injection station, cooled in the injection mold to a temperature above the orientation temperature, indexed to an interim station on said first core where the parison is preblown against the cold surfaces of an interim mold, which is larger than the shape of the original parison but smaller than the shape of the final desired article, cooled in the interim mold to the optimum orientation temperature, indexed on said first core to a blow station and positioned within the final blow mold whose cavity is in the shape of the final desired article. The preblown parison is then axially stretched in the closed final blow mold by extension of a poppet valve stem located within said first core rod, and finally radially expanded outward to its final shape, against the cavity walls of the blow mold, and then cooled to a suitable ejection temperature. After opening the blow molds, the core rod and biaxially oriented article are indexed to an ejection station, where the biaxially oriented article is removed. This method dispenses with the need for additional cores taught by the Valyi patents, but still requires the use of a pre-blow, or interim mold and interim mold station, with their attendant complexities and high costs. In this method there is some sacrifice of the capability to impart high levels of orientation, because the article is stretched less in going from the interim shape to the final shape, as compared to the stretching possible in other techniques wherein the parison is stretched at orientation temperature from the original parison shape to the final article shape.
In U.S. Pat. No. 4,065,246, Marcus teaches another injection blow molding process employing at least three stations wherein the parison is formed on a core in an injection mold, cooled to the desired orientation temperature range while in the injection mold, transferred on the same core to the final blow mold and allowed to dwell therein to bring the parison to uniform orientation temperature while the outer tip of the parison is in contact with a temperature controlled stop, and the remainder of the parison, except for the inner surface of its tip, is expanded slightly off the core to aid in the removal of the first core from said parison, by momentarily introducing low pressure air inside the parison. Alternatively, Marcus teaches that lubricant may be used to aid in removal of the first core from the parison. Next, the first core is removed from the partially expanded parison, and a second core is inserted and extended outwardly therein to stretch the parison longitudinally and thereby axially orient the parison. High pressure air is then introduced within the axially stretched parison to expand it radially outward until it contacts the cool surfaces of the blow mold cavity where it assumes its final shape and is cooled to a suitale ejection temperature. The biaxially oriented article is then transferred on said second core to an ejection station where it is ejected from the apparatus. This process avoids the duplication of molds but requires the duplication of cores and transfer means, and extra stations, all of which add complexities and additional costs.